1. Field
The present invention relates to a method of initializing an information recording medium by irradiating light beams, and particularly to a method for a rewritable phase-change optical recording layer that is capable of giving rise to a phase change to achieve repeated read-write operations by laser beam irradiation.
2. Description
Optical recording media have recently come into wide use as a viable information storage and archival means of large capacity. Among these media, demands have increased for a phase-change optical recording medium for use in a CD-R disk, for example, because of its repeatable (or writable) read/write capability.
The phase-change optical recording medium is generally provided on a disk-shaped substrate with several layers successively formed by, for example, the sputtering method, such as a first dielectric layer, a recording layer, a second dielectric layer and a layer either of metal or alloy, in the order recited. In addition, a layer composed of ultraviolet curing resin is formed thereon by spin coating, for example.
Since the thus formed recording layer is in an amorphous state, process steps of crystallization (i.e., xe2x80x9cinitializationxe2x80x9d steps) are to be carried out before recording onto the recording layer by a recording system or disk drive.
During the process steps, the recording layer is once heated to a temperature above its melting temperature. When the recording layer is then cooled relatively slowly, it transforms into a crystalline state. It transforms into an amorphous state when cooled rapidly.
As a suitable material for forming the substrate, polycarbonate resin is frequently used for its excellent properties such as mechanical stability, light weight and low costs, among others.
When the entire recording disk is initialized all at once, the allowable temperature of the polycarbonate substrate may be exceeded by heat from a large power density of light beam irradiation. Therefore, the initialization steps are generally carried out gradually over the disk area, in which disk portions are successively irradiated by a focused intense beam from a laser device along with the rotation of the disk.
To be more specific, the initialization of an optical disk is achieved by providing a laser device with its longer axis of the power distribution perpendicular to disk tracks and by imparting a gradual displacement of the laser device along the radial direction of the disk (i.e., perpendicular to the direction of disk tracks).
When a large fluctuation in the laser power exists along the longer axis of the laser power distribution perpendicular to disk tracks, reflectivity values of the optical disk after the initialization may undesirably fluctuate along the perpendicular direction. This may result in scatter in tracking signals, which is a drawback throughout succeeding reading and signal processing steps of the optical disk.
In addition, initialization steps using the above described background art laser device and optical system take a relatively long time to complete, since the optical disk is irradiated successively, portion-wise along with the rotation of the disk and cooled relatively slowly.
It is therefore an object of the present disclosure to provide improved method and apparatus of optical disk initialization, which overcome the above-noted difficulties.
It is another object of the present disclosure to provide a method of initializing a phase-change optical disk, especially realizing homogeneous crystallization and concomitant reflectivity of a recording layer over the optical disk in the direction perpendicular to disk tracks, thereby achieving excellent characteristics of recording and tracking signals throughout reading and signal processing steps with a disk drive system.
It is still another object of the present disclosure to provide a method of initializing a phase-change optical disk, capable of reducing the initialization time and thereby increasing the productivity of the optical disk.
The present inventors first examined closely initialization process steps of a phase-change optical disk as follows.
(1) The spatial distribution of a laser power used in the optical disk initialization affects considerably the characteristics of outputted data signals and of tracking signals.
(2) Regarding the spatial laser power distribution in the radial direction of the disk (i.e., perpendicular to the direction of disk tracks), signal characteristics of the optical disk after the initialization are found improved by bringing the laser power in both end regions of the spatial distribution to be equal to, or smaller than, that in the center region of the distribution.
Namely, since the spatial distribution of the laser power is generally elliptical-shaped. as will be described hereinbelow, by reducing the laser power in both edge regions of the elliptical spatial distribution in the major axis direction of the distribution ellipse, a homogeneous initialization can be achieved in the direction perpendicular to disk tracks.
(3) The amount of electrical current is known to be concentrated in the layer edge portions in an active layer and reflective layer in a background art laser device. This tends to increase the laser power in the edge regions along a longer axis direction of the spatial distribution. The above-noted increase in the laser power in the edge portions can be obviated by adequately polishing edge surfaces of at least one of an active layer and a reflective layer substantially perpendicular to the direction of the laser emission of the semiconductor laser device.
(4) When the laser power increase in the edge portions is found even after the polishing, this increase may be adjusted by further providing at least one optical unit such as, for example, a filter in the optical system, which is capable of attenuating the laser beams so as to achieve a laser power distribution appropriate for the disk initialization.
(5) The intensity of laser emission is known to decrease gradually with its operating time, in general. Therefore, by repeatedly turning on and off the laser energizing current within its allowable range either prior to incorporation into the optical system, or prior to the practical operations after the incorporation, the laser device can be made to have a laser power distribution with a decreased power in both edge regions of the spatial power distribution, which is adequately for use in the disk initialization.
(6) When an optical system is used for disk initialization incorporating the thus prepared laser device having a decreased power in the both edge regions of the spatial power distribution, a displacement step (or the amount of displacement relative to the disk track position in the direction perpendicular to the disk tracks) of an initialization head unit including the laser device per one disk rotation, is preferably larger than one half of, and smaller than, the width at half maximum of the spatial laser power distribution. This facilitates the initialization steps faster for the optical disks.
(7) In addition, the laser device to be incorporated into the optical system is found to preferably have laser emissions with the width at half maximum of at least 80 microns, of the spatial power distribution on the recording medium along the longer axis of the spatial distribution (or in the direction perpendicular to guide tracks). This also facilitates making the initialization steps faster for the optical disks.
According to the examination described above, the present disclosure is summarized as follows.
In order to achieve the foregoing and other objects, and to overcome the shortcomings discussed earlier, an improved method of initializing a phase-change optical information recording medium is provided, using an optical system incorporating a semiconductor laser device.
The laser device disclosed herein is characterized by its specified spatial power distribution. Namely, in the spatial distribution of laser power focused on the recording medium in the direction perpendicular to guide tracks, the laser device preferably has an average smaller in both end regions of the distribution, each of which corresponds to 10% of the width at half maximum of the distribution, than the average in the center region of the full width at half maximum of the distribution.
According to one aspect disclosed herein, edge surfaces substantially perpendicular to the direction of the laser emission, of at least one of an active layer and a reflective layer of the semiconductor laser device may preferably be polished such that an appropriate spatial laser power distribution such as mentioned above can be obtained.
According to another aspect disclosed herein, the optical system used in the disk initialization is further provided with at least one optical unit such as, for example, a filter which is capable of attenuating the laser beam intensity so as to achieve an appropriate laser power distribution.
According to still another aspect disclosed herein, the laser device may be cured for at least about six hours by being energized with at least about 80% of a maximum allowable electric power prior to the initialization of the recording medium so as to attain an appropriate laser power distribution.
According to another aspect disclosed herein, the displacement step of an initialization head unit including the laser device is preferably larger than one half of, and smaller than, the width at half maximum of the spatial laser power distribution.
According to another aspect disclosed herein, the laser device preferably has emitted light beams having a width at half maximum of at least 80 microns in the spatial power distribution focused on the recording medium in the direction perpendicular to guide tracks.
With the method and optical system incorporating the laser device disclosed herein, a homogeneous crystallization and concomitant reflectivity of the recording layer can be achieved in the optical disk through initialization steps. As a result, excellent characteristics of tracking signals and outputted data signals after the disk initialization can be achieved with thus initialized optical disks in a reduced initialization time, thereby increasing productivity of the optical disk.